1. Field of the Invention
The present invention relates to a new and improved method and apparatus for delivering fertilizer such as anhydrous ammonia to the soil through a plurality of knives penetrating the soil. More particularly, the invention relates to a system for controlling the flow of the anhydrous ammonia by controlling its temperature with an external refrigeration unit in cooperation with a heat exchanger through which the anhydrous ammonia passes and wherein its temperature is lowered by a refrigerated coolant also passing through the heat exchanger.
2. Description of the Relevant Art
Agricultural fertilizers are typically injected into the soil in a mixture of liquid and gaseous states through an apparatus that is pulled behind a motor driven tractor or the like, with the apparatus including a plurality of knife blades that penetrate the soil and have injection nozzles through which the fertilizer is injected or emitted into the soil. An ammonia storage tank is pulled behind the applicator and a hose connects the storage tank to a distribution system on the applicator, which directs the liquid and gaseous ammonia to the several knives on the applicator. It is, of course, desired that the ammonia fertilizer be applied uniformly over a field, but various factors can cause a lack of uniformity, depending upon the design of the applicator and the distribution system for the ammonia. The factors which affect the uniformity of the distribution of the ammonia over the field include ground speed, consistency of control of total flow from the storage tank, change of the ammonia temperature in the storage tank, partial depletion of ammonia in the storage tank, design of the distribution manifold system, the uniformity of flow paths to the knives, the effectiveness and consistency of flow control devices associated with the individual knives, and the interruption to flow of the knives such as might be caused by clogging at the outlet or nozzle of a knife.
Anhydrous ammonia in the storage tank is a saturated liquid at its vaporization temperature. As the liquid ammonia passes through a hose to the applicator, it experiences a pressure loss due to friction in the hose. Because of the lower pressure, some liquid ammonia vaporizes to cool the liquid to the vaporization temperature associated with that lower pressure. This drop in pressure changes the stream of ammonia into a mixture of liquid and gaseous phases. The greater the pressure drop, the greater is the ratio of gas to liquid. The mixture of liquid and gaseous phases has proven to be difficult to control, both as a total flow from the storage tank and when divided into the various lines leading to the plurality of knives carried by the apparatus. In spite of this problem, a majority of ammonia application systems in use continue to utilize systems wherein attempts are made to simultaneously control the liquid and gas phases.
The flow of ammonia through the system is generally controlled at three different locations. The total flow from the storage tank is controlled by a valve, pressure regulator, or metering pump at the end of the exit hose from the storage tank. The flow of ammonia to the individual knives on the apparatus is split at a manifold which divides the main flow of ammonia into a plurality of delivery hoses which are individually associated with a particular knife and its associated nozzle. The control of the flow of ammonia through the delivery hoses is typically accomplished by a restriction in the delivery line, which is normally an orifice of a predetermined diameter. Another point of control exists at the point where the ammonia exits the knife and is emitted into the soil. This control is again accomplished with an orifice or nozzle of a predetermined diameter. If the ammonia flowing through the system is never cooled, all control points (i.e., orifices, nozzles, valves, pumps or the like) change the ratio of gas to liquid and where control points reduce the pressure in the ammonia, the ratio of gas to liquid increases. A metering pump would have the opposite effect in that it would increase the pressure and thereby reduce the ratio of gas to liquid.
Distribution systems which attempt to avoid the mixed phases of ammonia have been devised and are commercially available. These systems typically divert a small portion of the liquid ammonia flowing from the storage tank and use it as a refrigerant to cool the main flow of the liquid from the storage tank to a temperature below its vaporization point. This is typically accomplished in a heat exchanger which mechanically separates the main stream into the smaller coolant stream and a resultant main stream.
There are two types of commercially available cooling systems for ammonia applicators. In the first system, the main stream of ammonia from the storage tank is cooled in a heat exchanger. A small side stream of liquid is diverted from the main stream after it has been cooled by the heat exchanger. This side stream is generally about 2% of the total stream. The side stream passes through a control valve and is reduced to a pressure close to atmospheric pressure. It becomes a stream of both gas and liquid at about -28.degree. F. This cold stream cools the incoming main stream of ammonia passing through the heat exchanger, primarily by the complete evaporation of that part of the side stream which is still liquid. After the main stream of sub-cooled ammonia leaves the heat exchanger, it goes to a valve or pump which is able to control its flow. A single flow measuring device is often put in the line before the flow control device so that the total flow to the injector knives can be monitored. Following the flow control and measuring devices, the main ammonia stream is split into several streams by a manifold with each of the several streams being delivered to one of the injector knives. Depending on the flow control device, ammonia leaving it may be in a mixed gas/liquid state before reaching the manifold. After the manifold, a device acting as an orifice is located in each delivery line to the knives to control the flow of ammonia to the knives. This orifice causes a large enough pressure drop so that the ammonia stream flashes into a mix of gas and liquid phases before reaching a knife. Ammonia vapor from the coolant side of the heat exchanger is typically split into two streams and these streams are individually sent to two of the injector knives in the system. The crop rows supplied by the knives receiving the split streams receive a somewhat higher amount of ammonia, resulting in a row-to-row variation of about 12%. Because all of the ammonia in the side streams leaves the heat exchanger as a vapor, any solid undissolved in the ammonia may be left in the heat exchanger. This could result in plugging of the cooling side of the exchanger caused by the deposit of solids dissolved in the ammonia.
In the second commercially available system, the main stream of ammonia from the storage tank is again cooled in a heat exchanger. After being cooled in the heat exchanger, the main stream of sub-cooled ammonia leaves the heat exchanger and goes through a valve or pump which is able to control its flow. A single flow measuring device is often put in the line just before the flow control device so that the total flow to the injector knives can be monitored. Following the flow control and measuring devices, the ammonia is split into several streams by a manifold with each of the streams passing to one of the individual injector knives on the apparatus. Depending on the flow control device, ammonia leaving it may be in a mixed gas/liquid state before reaching the manifold. After the manifold, a device acting as an orifice is located in each delivery line to the knives to provide controlled flow to each knife. This orifice causes a large enough pressure drop so that the ammonia stream flashes into a mix of gas and liquid phases before reaching a knife. Two of the several delivery lines pass through the heat exchanger before going to the knives. Liquid ammonia in these two lines cools the incoming main stream by additional partial evaporation. This results in a higher ratio of gas to liquid for those two lines compared to the lines which go directly to the knives and do not pass through the heat exchanger. The increased amount of gas reduces flow through the two lines because a given amount of gas causes a greater pressure drop than the same amount of liquid. The reduction of flow through the two lines used as coolant can be 10% or more. One advantage with this second commercially available system is that there is no potential for plugging by a solid dissolved in the ammonia because the liquid ammonia in the streams holds the solids in solution until exiting the knives.
As will be appreciated, neither of the aforedescribed commercially available systems uniformly delivers the ammonia to the soil through each of the knives and, accordingly, a system for improving the uniformity of the delivery of the ammonia would be well received in the agricultural community.